Energy error/hourglass error in a piston connecting rod crankshaft engine assembly Explicit Dynamics
To begin with, this is not a typical hourglass/energy error problem, so please don't tell me to search the database. I contacted my local Ansys Elite Channel Partner, but they couldn't find the cause of the problem.
I am simulating an engine assembly. The piston, the pins and the crankshaft are rigid bodies, while the connecting rod and the bolts are flexible. I use bonded contact for the threaded connection and frictional contacts for all other contacts.
The first problem is the hourglass energy. I am using the stiffness formulation (Flanagan-Belytschko) for hourglass damping. I tried different coefficients, but every time the hourglass damping is much larger than the internal energy.However, no hourglassing is visible. Mesh refinement didn’t work, on the contrary- on a very fine grid, the hourglass energy was about 1500% of the internal energy!! The viscous formulation is not working at all. I see that the hourglassing is accumulated in the bolts and near the bolt holes. The grid is perfect, almost 100% hexahedral elements. I use mass scaling, since there are a few smaller elements. Max element mass scaling is ~10.
The second problem is the energy error. I tried different meshes, different contact formulations, in vain. The energy error every time attains 1e6mJ, the shape of the energy error graph is always the same. At first I thought it could be somehow connected to the angular velocity on the crankshaft, since the RPM(t) is given by an asymptotic function, but this is just the physics of the task,
Please note the above energy graphs are from the fine grid calculations. It weights just too much, so I can attach a Workbench archive with the coarse mesh, I must clean the results too, otherwise it would weight 0.5GB.